The rapidity of modern-day data processing imposes severe demands on the ability to produce a printout record at very high speed. Impact printing, in which permanently shaped character elements physically contact a recording medium, are proving to be too slow, too bulky, and too noisy for many applications. Thus, the industry has turned to other alternatives involving non-impact printing schemes using various techniques to cause a desired character to be formed on the recording medium. Some of these involve the use of electrostatic or magnetic fields to control the deposition of a visible character-forming substance, either solid (i.e., dry powder) or liquid (i.e., ink) on the medium which is usually paper. Other systems utilize electrophotographic or ionic systems in which an electron or ion beam impinges on the medium and causes a change in coloration at the point of impingement. Still another system employs a thermal image to achieve the desired shape coloration change. Of more recent import is a printing technique, called ink jet printing, in which tiny droplets of ink are electronically caused to impinge on a recording medium to form any selected character at any location at high speed. Ink jet printing is a non-contact system which requires no specially treated recording media, ordinary plain paper being suitable, and which requires no vacuum equipment or bulky mechanisms. The present invention relates to this kind of printing system.
Ink jet systems may be classified as follows: (1) continuous, in which ink droplets are continuously spewed out from a nozzle at a constant rate under constant ink pressure; (2) electrostatic, in which an electrically-charged ink jet is impelled by controllable electrostatic fields; and (3) impulse, or ink-on-demand, in which ink droplets are impelled on demand from a nozzle by a controllable mechanical force. The invention is concerned with a nozzle head for this latter type of system.
Typical of the ink-on-demand systems is the approach set forth in U.S. Pat. No. 3,832,579 entitled PULSED DROPLET EJECTING SYSTEM. Here a cylindrical piezoelectric transducer is tightly bound to the outer surface of a cylindrical nozzle. Ink is delivered to the nozzle by means of a hose connected between one end of the nozzle and an ink reservoir. As the piezoelectric transducer receives an electrical impulse, it squeezes the nozzle which in turn generates a pressure wave resulting in the acceleration of the ink toward both ends of the nozzle. An ink droplet is formed when the ink pressure wave exceeds the surface tension of the meniscus at the orifice on the small end of the nozzle.
Another type of ink-on-demand printing is described in U.S. Pat. No. 3,179,042 entitled SUDDEN STEAM PRINTER. This system utilizes a number of ink-containing tubes, electric current being passed through the ink itself. Because of the high resistance of the ink, it is heated so that a portion thereof is vaporized in the tubes causing ink and ink vapor to be expelled from the tubes.
In the co-pending application, Ser. No. 292,841 entitled THERMAL INK JET PRINTER, filed Aug. 14, 1981 now abandoned by John L. Vaught et al. and assigned to the instant assignee, an ink-on-demand printing system is described which utilizes an ink-containing capillary having an orifice from which ink is ejected. Located closely adjacent to this orifice is an ink-heating mechanism which may be a resistor located either within or adjacent to the capillary. Upon the application of a suitable current to the resistor, it is rapidly heated. A significant amount of thermal energy is transferred to the ink resulting in vaporization of a small portion of the ink adjacent the orifice and producing a bubble in the capillary. The formation of this bubble in turn creates a pressure wave which propels a single ink droplet from the orifice onto a nearby writing surface or recording medium. By properly selecting the location of the ink-heating mechanism with respect to the orifice and with careful control of the energy transfer from the heating mechanism to the ink, the ink bubble with quickly collapse on or near the ink-heating mechanism before any vapor escapes from the orifice.
It is important to be able to monitor the condition and/or operability of the various nozzles in a thermal ink jet printhead system. It is highly desirable to detect in a very short period of time and almost instantly during the "printing" process whether any particular nozzle is operative or inoperative. It is further desirable to perform such a test during the printing operation itself with a minimum amount of delay to the operation. The need for detecting the nozzle failure is extremely critical in thermal jet print systems utilizing an array of nozzles which may total as many as 256 or more.